The present invention relates to a magnetic resonance imaging apparatus. More particularly, it relates to a magnetic resonance imaging apparatus that is suitable for an imaging performed by a multi-gradient echo type pulse sequence that aims at shortening the measurement time (i.e., for example, an echo planar spectroscopic imaging that allows space distribution information about the chemical shift to be measured at high speed).
The magnetic resonance imaging apparatus is an apparatus that performs the imaging in the following way: An object placed within a static magnetic field is irradiated with a radio frequency pulse with a specific frequency corresponding to a specific substance included in the object, thereby giving rise to a magnetic resonance phenomenon of the specified substance. Then, a magnetic resonance signal occurring from the object is utilized so as to image the chemical and physical information on the substance.
In particular, a method that has been known as a magnetic resonance spectroscopic imaging (MRSI) is as follows: A difference in the magnetic resonance frequency (hereinafter, referred to as a chemical shift), which is attributed to a dissimilarity in the chemical bond of a variety of molecules, is measured, thereby obtaining a space distribution image for each molecule (hereinafter, referred to as a chemical shift image).
Here, the magnitude of the chemical shift attributed to the dissimilarity in the chemical bond of the molecules is, usually, of the order of ppm (i.e., 1/1-millionth), which is exceedingly small. On account of this, in the MRSI, it becomes important to adjust the homogeneity of the magnetic field that exerts influences on the magnetic resonance frequency.
In general, one of the factors that exert considerable influences upon the homogeneity of the magnetic field is the existence of an object. This condition requires that the homogeneity of the magnetic field be enhanced in a state where the object is placed within the magnetic field. As a method of adjusting the homogeneity of the magnetic field, there has been proposed the following method (for example, in literatures such as xe2x80x9cJournal of Magnetic Resonancexe2x80x9d, Vol. 77, pp. 40-52 (1988)): Two kinds of values, i.e., the respective offset values of gradient magnetic fields in mutually different three directions and amounts of the electric currents to be flowed through the respective shimming coils, are varied so as to superimpose, on the static magnetic field, the magnetic fields that the respective gradient magnetic field generating coils generate and magnetic fields that the respective shimming coils generate, thereby adjusting the degree of the magnetic field homogeneity.
In this method, with a phantom employed as the subject to be measured, a reference image is scanned in advance that indicates current-magnetic field distribution characteristics of the respective gradient magnetic field generating coils and the respective shimming coils. Next, the object is inserted into the magnetic field, and a magnetic field distribution image within the object is measured as a target image of a subject to be shimmed. Then, the current-magnetic field distribution characteristics of the respective gradient magnetic field generating coils and the respective shimming coils are obtained from the reference image. Moreover, combinations of the distribution characteristics (values of the currents to be flowed through the respective coils) are calculated so as to uniform the magnetic field distribution within the object, then adjusting of the magnetic field is carried out on the basis of the combinations calculated.
As the reference image and the target image used here, a phase distribution image obtained by MRI (magnetic resonance imaging) is generally employed. In order to measure this phase distribution image, an imaging sequence called a spin echo pulse sequence is usually used. In particular, a pulse sequence is used in which the spin echo time and the gradient echo time are shifted by an amount of xcex94t.
In the conventional technique described in the above-mentioned literature, however, there exists a problem that no consideration has been given to the adjustment of the homogeneity of the magnetic field in the case where the multi-gradient echo type pulse sequence is used for the purpose of shortening the measurement time.
Namely, in the multi-gradient echo type pulse sequence, gradient polarity of a high-intensity gradient magnetic field is inverted continuously in a short period of time so as to measure echo train signals including a plurality of echo signals. This causes an eddy current of large amplitude to occur on such places as the inner surface of a bore within a magnet or the surface of the object. Accordingly, there exists a serious problem that a magnetic field induced by the eddy current makes the magnetic field distribution inhomogeneous at the detection time of the echo signals. However, in the spin echo pulse sequence in the conventional technique with which the reference image is measured, one echo signal is measured with a single excitation, and it is not repeated to invert the polarity of high-intensity gradient magnetic field in a short period of time. On account of this, there appears no influence of the eddy current upon the reference image data obtained. This results in a problem that it is impossible to perform an adjustment in which the eddy current has been taken into consideration.
Incidentally, there are many cases where, in order to cancel out the influences of the eddy current, a gradient coil equipped with an active shield is used. It is difficult, however, to completely cancel out the eddy current occurring on the inner surface of the magnet bore, and thus the active shield exhibits no effect of eliminating the eddy current on the object surface. Consequently, it has been required to execute the adjustment of the degree of the magnetic field homogeneity in which the eddy current has been taken into consideration.
It is an object of the present invention to improve the adjustment of the homogeneity of the magnetic field in the magnetic resonance imaging apparatus that uses the multi-gradient echo type imaging pulse sequence.
A representative configuration (a first configuration) of a representative magnetic resonance imaging apparatus according to the present invention includes a static magnetic field generating unit for generating a static magnetic field to be applied to an object, a shimming magnetic field generating unit (shimming coil) for generating a N-channel (Nxe2x89xa73) shimming magnetic field that adjusts the degree of the static magnetic field homogeneity, a gradient magnetic field generating unit (gradient magnetic field generating coil) for generating first, second and third directions of gradient magnetic fields to be applied to the object, a radio frequency pulse generating unit (radio frequency pulse generating coil) for generating a radio frequency pulse to be applied to the object, a signal detecting unit for detecting a magnetic resonance signal generated from the object, a computing unit for performing computation in accordance with the magnetic resonance signal detected by the signal detecting unit, a pulse sequence controlling unit for controlling operations of the shimming magnetic field generating unit, the gradient magnetic field generating unit, the radio frequency pulse generating unit and the signal detecting unit, and a static magnetic field homogeneity degree-adjusting unit for controlling the shimming magnetic field generating unit and/or the gradient magnetic field generating unit so as to adjust the degree of the static magnetic field homogeneity.
Moreover, the pulse sequence controlling unit executes a static magnetic field-adjusting pulse sequence control that causes the magnetic resonance signal to be detected while applying at least one direction of gradient magnetic field from among the gradient magnetic fields with gradient polarity of the one direction of gradient magnetic field being inverted periodically. In addition, the computing unit reconstructs a magnetic resonance spectrum from the magnetic resonance signal and extracts a peak of the frequency component, the magnetic resonance signal being detected by the static magnetic field-adjusting pulse sequence control, the peak corresponding to a specific nucleus. Furthermore, the static magnetic field homogeneity degree-adjusting unit adjusts the degree of the static magnetic field homogeneity so that the width of the extracted peak becomes narrow.
Explaining the above-mentioned description in more detail, the pulse sequence controlling unit performs the control explained in the following (i.e., the static magnetic field-adjusting pulse sequence control): An exciting radio frequency pulse is applied simultaneously with the gradient magnetic field of the third direction so as to selectively excite nuclear spins contained in slices or volumes in the third direction. After that, after a time set in advance has elapsed, an inverting radio frequency pulse is applied simultaneously with the gradient magnetic field of the third direction. Next, the reading-out gradient magnetic field of the first direction is applied with the gradient polarity thereof being inverted periodically, thereby generating echo train signals including the chemical shift and the spatial information in the first direction and then detecting the echo train signals.
Subsequently, the computing unit, from the echo train signals detected by the static magnetic field-adjusting pulse sequence control, determines a magnetic resonance spectroscopic image involving 2-dimensional information on the chemical shift and the spatial information in the first direction. Then, the computing unit creates an integrated spectrum over the entire slices or volumes by adding the spectra of reading-out points in the first direction. Moreover, the computing unit extracts, from the integrated spectrum, a peak corresponding to a hydrogen nucleus in a water molecule, then calculating the width of the peak. Furthermore, the static magnetic field homogeneity degree-adjusting unit adjusts the degree of the static magnetic field homogeneity so that the width of the calculated peak becomes narrow.
When the computing unit obtains at least one of the magnetic resonance image, the magnetic resonance spectrum and the magnetic resonance spectroscopic image, prior to the measurements of the magnetic resonance image, the magnetic resonance spectrum and the magnetic resonance spectroscopic image, the configuration as described above permits the pulse sequence controlling unit to periodically invert the gradient polarities of the gradient magnetic fields to be applied to the object or the phantom. This makes it possible to detect the multi-gradient echo type magnetic resonance signal subjected to the influences of the eddy current. Moreover, the computing unit reconstructs the magnetic resonance spectrum from the magnetic resonance signal, then extracting the peak of the frequency component corresponding to the specific nucleus. Furthermore, the static magnetic field homogeneity degree-adjusting unit performs the adjustment so that the width of the peak becomes narrow, i.e., becomes steep. This adjustment improves the degree of the static magnetic field homogeneity.
At this time, in the case of the MRSI that measures the chemical information on a living body, it is desirable to select, as the specific nucleus, a nucleus contained in the water (for example, the hydrogen nucleus).
More concretely, in the above-described configuration, the following is preferable: The static magnetic field homogeneity degree-adjusting unit, in accordance with the peak width, changes an offset current value for the gradient magnetic field of one direction by a certain fixed amount. After that, it is repeated a set number of times to perform a control of causing the pulse sequence controlling unit and the computing unit to execute the measurement of the peak width again, thereby searching and determining an optimum offset current value so that the peak width becomes the narrowest. In this case, after the optimum offset current value for the gradient magnetic field of one direction has been searched and determined, it is preferable to search and determine, in sequence, optimum offset current values for the gradient magnetic fields of other directions.
Also, in substitution for or in combination with the above-described operation of adjusting the degree of the static magnetic field homogeneity through the gradient magnetic fields, it is possible to adjust current values of the respective shimming coils of the shimming magnetic field generating unit including the plurality of shimming coils. In this case, the static magnetic field homogeneity degree-adjusting unit, in accordance with the detected peak width, changes only a value of a current to be flowed through one channel of shimming coil. After that, it is repeated a plurality of times to execute the static magnetic field-adjusting pulse sequence control and to execute the calculation of the peak width, thereby determining an optimum value of the current to be flowed through the above-described one channel of shimming coil so that the peak width becomes the narrowest. In addition, after determining the optimum value of the current to be flowed through the above-described one channel of shimming coil, the static magnetic field homogeneity degree-adjusting unit determines, in sequence, optimum values of currents to be flowed through the plurality of channels of shimming coils other than the above-described one channel of shimming coil.
In this way, the pre measurement (pre scan) is executed that is to be executed prior to the main measurement (main scan) of the magnetic resonance spectroscopic image. Accordingly, it becomes possible to set, in a short while and with excellent reproducibility, the optimum offset current values for the gradient magnetic fields of the three directions and/or the optimum values of the currents to be flowed through the shimming coils for generating the plurality of channels of shimming magnetic field so that the peak width becomes the narrowest.
A representative configuration of a magnetic resonance imaging method according to the present invention includes a step of generating a static magnetic field to be applied to an object, a step of generating a N-channel (Nxe2x89xa73) shimming magnetic field for adjusting the degree of the static magnetic field homogeneity, a step of generating the gradient magnetic fields of the first, second and third directions to be applied to the object, a step of generating a radio frequency pulse to be applied to the object, a step of detecting a magnetic resonance signal generated from the object, a computing step of performing computation in accordance with the magnetic resonance signal detected by a signal detecting unit, a pulse sequence controlling step of controlling operations of a shimming magnetic field generating unit, a gradient magnetic field generating unit, a radio frequency pulse generating unit and the signal detecting unit, and a static magnetic field homogeneity degree-adjusting step of controlling the shimming magnetic field generating unit and/or the gradient magnetic field generating unit so as to adjust the degree of the static magnetic field homogeneity.
Moreover, at the pulse sequence controlling step, a static magnetic field-adjusting pulse sequence control is executed that detects the magnetic resonance signal while applying a gradient magnetic field of at least one direction of the three directions with gradient polarity of the gradient magnetic field of one direction being inverted periodically. At the computing step, a magnetic resonance spectrum is reconstructed from the magnetic resonance signal and a peak of the frequency component is extracted, then calculating width of the peak, the magnetic resonance signal being detected by the static magnetic field-adjusting pulse sequence control, the peak corresponding to a specific nucleus. Furthermore, at the static magnetic field homogeneity degree-adjusting step, the degree of the static magnetic field homogeneity is adjusted so that the width of the peak becomes narrow.
Explaining the above-mentioned description in more detail, at the pulse sequence controlling step, the control explained in the following (i.e., the static magnetic field-adjusting pulse sequence control) is executed: First, an exciting radio frequency pulse is applied simultaneously with the gradient magnetic field of the third direction so as to selectively excite nuclear spins contained in slice or volume in the third direction. After that, after a time set in advance has elapsed, an inverting radio frequency pulse is applied simultaneously with the above-described gradient magnetic field of the third direction. Next, the reading-out gradient magnetic field of the first direction is applied with the gradient polarity thereof being inverted periodically, thereby generating echo train signals including the chemical shift and the spatial information in the first direction and then detecting the echo train signals.
Subsequently, at the computing step, from the echo train signals detected by the static magnetic field-adjusting pulse sequence control, a magnetic resonance spectroscopic image is reconstructed that has 2-dimensional information on the chemical shift and the spatial information in the first direction. Then, an integrated spectrum over the entire slice or volume is created by adding the spectra of the reading-out points in the first direction. Moreover, a peak corresponding to a hydrogen nucleus in a water molecule is extracted from the integrated spectrum, then the width of the peak is calculated. Furthermore, at the static magnetic field homogeneity degree-adjusting step, the degree of the static magnetic field homogeneity is adjusted so that the peak width determined by the calculation becomes narrow.
In this way, in the magnetic resonance imaging method according to the present invention, the pre measurement (pre scan) is executed. Accordingly, it becomes possible to set, in a short while and with excellent reproducibility, optimum offset current values for the gradient magnetic fields of the three directions and/or optimum values of currents to be flowed through shimming coils for generating a shimming magnetic field of a plurality of channels so that the peak width becomes the narrowest. Namely, with the eddy current taken into consideration, the degree of the static magnetic field homogeneity is adjusted. This makes it possible to solve the serious problem that a magnetic field, which the eddy current induces, makes inhomogeneous the magnetic field distribution at the time of detecting the echo signals. Consequently, it becomes possible to measure, in a short while, a chemical shift image with less inhomogeneity of the static magnetic field.
Incidentally, in substitution for or in combination with the above-described operation of adjusting the degree of the static magnetic field homogeneity through the gradient magnetic fields, it is possible to adjust current values of the respective shimming coils of the shimming magnetic field generating unit including the plurality of shimming coils. In this case, in the static magnetic field homogeneity degree-adjusting unit in the above-described magnetic resonance imaging apparatus, when searching and determining a value of an i-optimum current to be flowed through an i-channel (i=1, 2, . . . , N) shimming coil, the search and the determination are performed in the following way: In the case where a value of a j-optimum current to be flowed through a j-channel (j=1, 2, . . . , N, where jxe2x89xa0i) shimming coil has been already searched and determined, a current the value of which is equal to the value of the j-optimum current is flowed through the j-channel (j=1, 2, . . . , N, where jxe2x89xa0i) shimming coil. In the case where the value of the j-optimum current to be flowed through the j-channel (j=1, 2, . . . , N, where jxe2x89xa0i) shimming coil has not been searched and determined yet, the search and the determination are performed in the following way: In a state where no current is flowed through the j-channel (j=1, 2, . . . , N, where jxe2x89xa0i) shimming coil, in accordance with the peak width, only a value of a current to be flowed through the i-channel (i=1, 2, . . . , N) shimming coil is changed. After that, it is repeated a plurality of times for the pulse sequence controlling unit to execute the static magnetic field-adjusting pulse sequence control, and it is repeated a plurality of times for the computing unit to execute the calculation of the peak width. This, thereby, makes it possible to search and determine in sequence the value of the i-optimum current to be flowed through the i-channel (i=1, 2, . . . , N) shimming coil so that the peak width becomes the narrowest.
Also, at the static magnetic field homogeneity degree-adjusting steps in the above-described magnetic resonance imaging method, when searching and determining a value of an i-optimum current to be flowed through an i-channel (i=1, 2, . . . , N) shimming coil, the search and the determination are performed in the following way: In the case where a value of a j-optimum current to be flowed through a j-channel (j=1, 2, . . . , N, where jxe2x89xa0i) shimming coil has been already searched and determined, a current the value of which is equal to the value of the j-optimum current is flowed through the j-channel (j=1, 2, . . . , N, where jxe2x89xa0i) shimming coil. In the case where the value of the j-optimum current to be flowed through the j-channel (j=1, 2, . . . , N, where jxe2x89xa0i) shimming coil has not been searched and determined yet, the search and the determination are performed in the following way: In a state where no current is flowed through the j-channel (j=1, 2, . . . , N, where jxe2x89xa0i) shimming coil, in accordance with the above-mentioned peak width, only a value of a current to be flowed through the i-channel (i=1, 2, . . . , N) shimming coil is changed. After that, it is repeated a plurality of times at the pulse sequence controlling step to execute the static magnetic field-adjusting pulse sequence control, and it is repeated a plurality of times at the computing step to execute the calculation of the peak width. This, thereby, makes it possible to search and determine in sequence the value of the i-optimum current to be flowed through the i-channel (i=1, 2, . . . , N) shimming coil so that the peak width becomes the narrowest.
Moreover, in the above-described explanation, the following conditions can be suitably selected: Employing the half-value width as the peak width, employing a peak width corresponding to a value that is equal to 20% of the peak maximum intensity, and so on.
The following is the summary of a representative invention in the present invention given by referring to FIG. 1:
Prior to measurements of the magnetic resonance image, the magnetic resonance spectrum and the magnetic resonance spectroscopic image, the pulse sequence controlling unit periodically inverts gradient polarities of the gradient magnetic fields to be applied to an object or a phantom. This makes it possible to measure the multi-gradient echo type magnetic resonance signal subjected to influences of the eddy current. Then, from the magnetic resonance spectrum obtained in accordance with the magnetic resonance signal, a peak of the frequency component is extracted that corresponds to a nucleus contained in the water. Next, the static magnetic field homogeneity degree-adjusting unit performs the adjustment so that width of the peak becomes narrow, i.e., becomes steep, thereby improving the degree of the static magnetic field homogeneity.
As having been explained so far, the present invention makes it possible to improve the adjustment of the homogeneity of the magnetic field in the magnetic resonance imaging apparatus that uses the multi-gradient echo type imaging pulse sequence. As a result, it becomes possible to measure, in a short while, the chemical shift image with no influence of the inhomogeneity of the magnetic field.